Pollution and climate change have led to unprecedented global environmental changes including decreases in air, soil and water quality.

In recent decades, concentrations of Dissolved Organic Carbon (DOC) have tended to increase in soils and surface waters in many northern parts of the globe. DOC gives some stream water a brown colour.

[Image: Andrew Sier, NERC Centre for Ecology & Hydrology]

This has raised concerns for public health, water treatment costs and the fate of carbon stocks.

[Image: Andrew Sier, NERC Centre for Ecology & Hydrology]

Soils are major stores of carbon, which, if released in the form of DOC may eventually return to the atmosphere in the form of greenhouse gases like carbon dioxide and methane. Keeping carbon 'locked' in soils is an important response to climate change.

[Image: Andrew Sier, NERC Centre for Ecology & Hydrology]

Climate change, land use change, and nitrogen pollution may all lead to rising DOC concentrations. Increasing DOC may also be linked to the recovery of soils from acidification (from 'acid rain').

[Image: Natural Environment Research Council]

This UK study suggested that most of the observed increases in DOC that occurred prior to 2005 were due to recovery from acidification. This has important implications for how DOC concentrations are managed.

* INCLUDES 'WHY IT MATTERS' FEATURE * This paper, published in a Special Issue of the journal Ecological Indicators to mark 20 years of data collection at ECN terrestrial sites, presents an analysis of the temporal relationships between soil solution chemistry and parameters thought to regulate Dissolved Organic Carbon (DOC) production. Data from both ECN and Forest Level II plots were used.

Long-term monitoring of surface water quality has shown increasing concentrations of dissolved organic carbon (DOC) across a large part of the Northern Hemisphere. Several drivers have been implicated including climate change, land management change, nitrogen and sulphur deposition and CO2 enrichment. Analysis of stream water data, supported by evidence from laboratory studies, indicates that an effect of declining sulphur deposition on catchment soil chemistry is likely to be the primary mechanism, but there are relatively few long term soil water chemistry records in the UK with which to investigate this, and other, hypotheses directly. In this paper, we assess temporal relationships between soil solution chemistry and parameters that have been argued to regulate DOC production and, using a unique set of co-located measurements of weather and bulk deposition and soil solution chemistry provided by the UK Environmental Change Network and the Intensive Forest Monitoring Level II Network. We used statistical non-linear trend analysis to investigate these relationships at 5 forested and 4 non-forested sites from 1993 to 2011. Most trends in soil solution DOC concentration were found to be non-linear. Significant increases in DOC occurred mostly prior to 2005. The magnitude and sign of the trends was associated qualitatively with changes in acid deposition, the presence/absence of a forest canopy, soil depth and soil properties. The strongest increases in DOC were seen in acidic forest soils and were most clearly linked to declining anthropogenic acid deposition, while DOC trends at some sites with westerly locations appeared to have been influenced by shorter-term hydrological variation. The results indicate that widespread DOC increases in surface waters observed elsewhere, are most likely dominated by enhanced mobilization of DOC in surficial organic horizons, rather than changes in the soil water chemistry of deeper horizons. While trends in DOC concentrations in surface horizons have flattened out in recent years, further increases may be expected as soil chemistry continues to adjust to declining inputs of acidity.